For driver debugging purposes these ioctls allow test applications to
access hardware registers directly. Regular applications must not use
them.

Since writing or even reading registers can jeopardize the system
security, its stability and damage the hardware, both ioctls require
superuser privileges. Additionally the Linux kernel must be compiled
with the CONFIG_VIDEO_ADV_DEBUG option to enable these ioctls.

To write a register applications must initialize all fields of a struct
v4l2_dbg_register except for size and
call VIDIOC_DBG_S_REGISTER with a pointer to this structure. The
match.type and match.addr or match.name fields select a chip
on the TV card, the reg field specifies a register number and the
val field the value to be written into the register.

To read a register applications must initialize the match.type,
match.addr or match.name and reg fields, and call
VIDIOC_DBG_G_REGISTER with a pointer to this structure. On success
the driver stores the register value in the val field and the size
(in bytes) of the value in size.

When match.type is V4L2_CHIP_MATCH_BRIDGE, match.addr
selects the nth non-sub-device chip on the TV card. The number zero
always selects the host chip, e. g. the chip connected to the PCI or USB
bus. You can find out which chips are present with the
ioctl VIDIOC_DBG_G_CHIP_INFO ioctl.

When match.type is V4L2_CHIP_MATCH_SUBDEV, match.addr
selects the nth sub-device.

These ioctls are optional, not all drivers may support them. However
when a driver supports these ioctls it must also support
ioctl VIDIOC_DBG_G_CHIP_INFO. Conversely
it may support VIDIOC_DBG_G_CHIP_INFO but not these ioctls.

VIDIOC_DBG_G_REGISTER and VIDIOC_DBG_S_REGISTER were introduced
in Linux 2.6.21, but their API was changed to the one described here in
kernel 2.6.29.

We recommended the v4l2-dbg utility over calling these ioctls directly.
It is available from the LinuxTV v4l-dvb repository; see
https://linuxtv.org/repo/ for access
instructions.